If a voltage higher than a reference voltage of a unit battery (cell) is needed such as in a hybrid vehicle which uses a lithium ion cell as a power source, it is typical to connect a plurality of unit batteries in series. However, even though the batteries produced via the typical manufacturing method have the same structure using the same anode, cathode and electrolyte, there is existed a difference in charge or discharge characteristics between each battery connected in series.
Therefore, since a voltage difference exists between the unit batteries when using the batteries connected in series, there are problems in that a total voltage (total voltage of batteries connected in series) becomes zero so that a recharge is needed even though one battery is totally discharged irrespective of voltage of the other batteries in the unit batteries connected in series, the battery reaching the certain voltage at first is overcharged due to different voltage between each battery even at the time of the recharge, and the battery which still does not reach any certain voltage exists even though some batteries are overcharged.
Further, if the number of charging/discharging times is higher, significant degradation can occur in materials composing the battery so that characteristics of the battery become different and such degradation is responsible for aggravating the difference between the individual cells.
Therefore, in order to address such problems, various charge equalization apparatuses are actively proposed to achieve charge equalization of the batteries connected in series.
As an example, Korean Patent Laid-Open No. 2003-0096978 is directed to a system which is consisted of a plurality of unit cells, a charging means, a discharging means, and serial-parallel conversion switch, and performs charging by discharging the plurality of unit cells equally and then connecting the discharged unit cells in series using the serial-parallel conversion switch. Korean Patent Laid-Open 2007-0064244 is directed to a system which comprises a cell unit, a field-effect transistor unit connected to the cell unit, an amplifying unit connected to the field-effect transistor unit, a multiplexer controlling an output signal of the amplifying unit, a comparator comparing and determining a variation in a voltage signal of the cell unit, an A/D converting unit converting the output from the comparator into a digital signal, a Micom unit inputted with the signal outputted from the A/D converter and outputting a signal corresponding to the charging/discharging condition, a switching unit supplying the battery equalization current in accordance with the signal of the Micom unit, and a known charging/discharging circuit.
Further, Japanese Patent Laid Open No. 1998-032936 is directed to a system which is consisted of a plurality of unit cells, a detection means for detecting remaining capacity in each unit cell, a charge replacement means and a discharge replacement means for performing charging and discharging in each unit cell, a control means for controlling charging and discharging in each unit cell separately, and a direct current/direct current converter performing charging and discharging separately in each unit cell. Japanese patent Laid Open No. 2004-194410 is directed to a system which is consisted of two or more unit cell groups, a current difference detection means for detecting a difference in currents flowing in each of a first cell group and a second cell group, and a means for controlling charging/discharging current in the cell group based on the difference in currents.
However, since the prior charge equalization apparatus has a charge equalization apparatus provided in each of the batteries connected in series so that charging or discharging is performed separately on each battery, there are problems that complexity and volume of the charge equalization apparatus are increased and thus productivity is reduced and production cost is higher, and components composing the charge equalization apparatus or the switch module for controlling it must endure higher voltage stress.
In order to address such problems, the present applicant have proposed a scheme of the charge equalization control by charging individual battery using two-stage DC-DC converter in Korean Patent Application No. 10-2007-0104033. The charge equalization apparatus using the two-stage DC-DC converter according to the prior patent application is such that each battery module composing the battery string shares the DC-DC converter using a charge control switch in order to reduce the number of DC-DC converters. However, since each battery cell can control a charging current only if the charge control switch must be used for all of a positive direction and a negative direction of the battery whenever the charge control switch is used for each battery cell, the number of the switches typically becomes double the number of batteries. This is considerably disadvantageous in terms of cost and volume of products consisted of multiple battery cells. Therefore, there is a need for method that can reduce the number of charge control switches used to form a charge path of each battery cell.